Conventional centrifugal pumps typically comprise an impeller mounted to a motor shaft, a liquid to be pumped entering the impeller along its axis of rotation. The impeller is provided with blades that force the liquid radially toward the impeller circumference, thereby discharging the liquid at a relatively high speed into a volute formed within a casing surrounding the impeller. The centrifugal force generated by rotation of the impeller accelerates the liquid to a high speed. In turn, the dynamic pressure associated therewith is converted to static pressure in the volute, where the speed is gradually reduced. A significant limitation associated with centrifugal pumps, as well as other pumps that do not provide volumetric displacement of liquid, is that, at start-up when the pump is empty, such pumps are unable to remove a significant quantity of air from the suction pipe. Hence, auxiliary devices must be provided that allow the pumps to be primed, after which time they will function normally.
When a centrifugal pump is installed at a level higher than that of the tank from which the liquid is to be taken, one way to provide self-priming has been to maintain a sufficient quantity of liquid at the impeller entry during the self-priming phase or, alternatively, to pass it through an appropriate opening in the volute casing. This approach, however, usually requires that a tank be provided to contain liquid for the priming process. It is also characterized by a relatively low self-priming height and an unavoidable pressure drop, since the openings needed for the priming process also remain active during pump operation. This type of priming has been used primarily for agricultural applications.
Another way to insure self-priming of a centrifugal pump has been to use an auxiliary vacuum pump. While such an arrangement has generally been found efficient, it has also has been costly.
A further approach has been to incorporate an ejector between the suction mouth and the impeller entry. In pumps of this type, or so-called “jet pumps”, the pump body is filled with liquid during pump start-up so as to also fill the ejector. After the pump has been started-up, the impeller induces circulation of liquid through the ejector that entrains gas, thus forming a liquid-gas mixture of from which the gas will separate in the upper portion of the pump body. Recirculation of the liquid-gas mixture continues until the gas has been eliminated. Thereafter, the pump will function in a normal manner.
Although jet pumps are considered easy to use and have been found less costly to install than other systems, their performance has been greatly impeded both by the section restriction of the ejector installed in the suction conduit and by the pressure drop that results from an orifice situated in front of the Venturi tube, which remains active even when the pump is in operation. While pumps having no self-priming devices are generally more efficient than jet pumps, their operation is often hindered by the possible presence of air bubbles or pockets in the suction pipe. In situations where it is essential for the pump to operate with reasonable continuity, these pumps require accessories, auxiliary systems or the continuous presence of supervisory personnel.
There are also applications where it is important that the pump be mobile (e.g., motor pumps for agriculture, fire-fighting services, road tankers, and emergencies of various kinds) and, hence, require a self-priming system that is not only fast and reliable, but also highly efficient.